The global market for hunting lead shot is currently 120,000 tonnes per annum. In Europe, it is about 60,000 tonnes.
As a result of stricter environmental regulations, the use of lead in ammunition for hunting and target shooting is already banned in several European countries (especially in the Netherlands, Sweden and Germany), in particular for shooting in wetlands.
Among the possible and economically viable technical solutions for substitutes for hunting lead shot (iron, steel, heavy metals, alloys, ceramics, etc.), only steel shot appears to be a viable alternative. Indeed, it has satisfactory density, is non-toxic, comes to an affordable price, etc.
For about twenty years, steel shot has also been marketed in North America as a substitute charging by the biggest manufacturers in the sector.
It is known that the charging of a cartridge comprises spherical balls that not only have a diameter tolerance of about 0.1 mm but also a narrow weight tolerance in order to guarantee reproducible ballistic behaviour and to ensure constant charging of the cartridge, which is done by volumetric measurement. The steel balls must be subjected to a surface treatment such as copper plating, addition of graphite, etc. in order to prevent any aggregation of the charging, which could adversely affect good dispersion.
The calibres of the most frequently used charges are shown in Table 1 below.
For this type of application, a low-alloy soft-steel type of composition such as for example: max. 0.06% C, max. 0.4% Mn, max. 0.1-0.3% Si, max. 0.04% P, max. 0.04% S, is sought.
The hardness must be as low as possible, not exceeding 110 HV 10 at the surface and 100 HV 10 at the core.
Currently, from the purely ballistic point of view, hunters and marksmen have a clear preference for the lead shot that is traditionally used. Indeed, they consider that lead shot has some specific ballistic features that are difficult to find in substitute products.
The traditional market of hunters and marksmen thus demonstrate some reluctance to use the steel ball cartridges currently on the market. The main arguments to justify this reluctance towards steel balls are associated with the following aspects:                increased risk of ricochets,        greater wear of the barrel,        greater loss of speed and penetration as a function of distance,        higher ammunition price.        
Other non-toxic alloys (for example “Hevi-Shot”: tungsten-nickel-iron) have also been developed. They can technically compete with steel shot since, due to their high density, they provide a ballistic behaviour that is similar to that of traditional lead charging. However, these alloys are too expensive.
On the other hand, with regard to the production of steel shot, there are two possible methods.
From Molten Steel
Given the tonnages to be produced, this process entails setting up the method on premises for manufacturing molten steel with the capacity of a foundry type and supplementing it with a suitable granulation unit. Various techniques for the granulation of molten metal are available on the market:                the Osprey method, gaseous atomisation under pressure, does not really fall within the “granulation” type since it is dedicated to the production of fine particles (100 μm) intended for powder metallurgy;        water granulation: a jet of steel is cast under a high-pressure water jet or directly into a trough. The size of the particles obtained may vary (from one to several millimeters depending on the technique) but the common feature of these is the irregular, non-spherical shape of the product;        steam granulation: this method, developed in particular by Mintek (South Africa), uses a jet of steam, which allows to avoid an excessively violent mechanical disturbance of the metal stream.        
This type of method entails a wide size distribution as a result of the absence of spherical shape, which leads to the loss of part of the production (yield of the order of 60%). Moreover, in contrast to lead, which has a much lower melting point than steel, it is difficult to re-melt the steel balls produced outside the calibre in a closed circuit.
The best results are obtained with a steel with a very high carbon content (>1.5%) whose hardness is lower than 100 HV 10 or even 80 HV 10. ULC steels are not to be used in the frame of this technique.
To reduce the size distribution, some methods precede the water granulation with a system for dividing the flow of molten metal. The latter is for example spread over a rotating plate before it is subjected to final water or steam granulation.
From Wire or Sheet (Mechanical Method)
This method is inspired by the techniques for manufacturing nails or other high-volume pieces of steel. Striking and stamping operations on drawn wire or on sheet allow to obtain balls. Machines are known that allow to manufacture steel ball for bearings. From a reel of steel wire, a steel blank is cut for subsequent and progressive transformation into balls by deformation between grooved, generally cast, plates.
The wire used is of a ULC type, for example with a carbon content of 0.02% C. This type of wire is very expensive and not at all readily available if one bears in mind the quantities to be produced in the sector of hunting cartridges. On the other hand, the advantage of wire is that the yield from production is 100%.
With flat steels, soft steels are much more readily available, including in various thicknesses thus corresponding to various ball diameters. Unfortunately, given the waste, part of the production is also unusable in this case.
From Iron Powder
Powder metallurgy is used to manufacture some pieces, usually of a complex form, from metal materials. This consists in compacting the powder in a mould and subjecting the whole to a high-temperature sintering operation.
Some tungsten carbide spheres are in particular prepared by this technique. Starting with iron powder, one may envisage to examine the possibility of obtaining extra-soft steel balls by a compaction/sintering method.
To summarise, the techniques for obtaining balls from wire, sheet and iron powder have the disadvantage of being more expensive than the molten technique.
State of the Art
In the state of the art, a current economical method is known for softening by decarburisation a carbon steel such as a large-crystal ferrite. Let us recall that a single-crystal pure iron has extremely low hardness (30-40 HV 10), which may be considered as the absolute lower limit.
Document WO-A-00/44517 proposes a steel projectile and associated manufacturing method with relatively high carbon content (up to 1.5% C), produced by water atomisation and softened by annealing under non-oxidising atmosphere at controlled dew point, at a temperature between 600 and 1,200° C., in order to make it suitable for ballistic use. The softening mainly results from surface decarburisation, preferably with an average Knoop surface hardness lower than 225 (Vickers surface hardness at least higher than 130).
Aims of the Invention
The present invention aims to provide a solution that does not have the drawbacks of the state of the art.
In particular, the invention aims to provide a treatment based on a common grade of steel allowing to obtain a final hardness lower than 100 HV 10.
In addition, one aim of the invention is to provide this treatment with a view to obtaining low hardness in steel balls, uniformly across their entire volume (at the core).
An additional aim of the invention is to eliminate a certain tendency to ricochet, which is commonly inherent to steel projectiles.
A further aim of the invention is to allow the manufacture of balls with a good spherical shape.
Yet another additional aim of the invention is to manufacture steel balls with (self)-lubricating properties that allow reduced wear of the barrel of the weapon.
Main Characteristic Elements of the Invention
A first aim of the present invention relates to a method for manufacturing a projectile made of steel or based on iron for cartridges for hunting or target shooting with a view to give it a hardness between 40 HV 10 and 110 HV 10 at room temperature, preferably between 40 HV 10 and 85 HV 10, characterised by at least the following successive steps:                liquid steel or pig iron, hereafter called molten metal, whose carbon content is between 0.8 and 4.0% C by weight, preferably between 0.8 and 2.0% C by weight, is used;        the molten metal is cast according to a water or steam granulation method in order to obtain spheroid steel particles;        said particles are subjected to a heat treatment for softening the solidified metal by graphitisation while remaining in the ferritic range;        said particles are possibly subjected to a surface coating treatment.        
According to the invention, the softening heat treatment comprises at least the following successive steps:                said metal is annealed for about 3 minutes from room temperature up to a temperature higher than 800° C.;        said steel is cooled in water, preferably in boiling water, at a speed of at least 20° C./s down to room temperature;        said metal is tempered under non-oxidising or slightly reducing atmosphere of HNx type to a temperature below 700° C., preferably below 650° C.;        said metal is maintained under the above-mentioned atmosphere at this last temperature for a period of between 1 and 6 days, preferably between 1 and 4 days.        
Maintaining the method of the invention under non-reducing atmosphere is mandatory to prevent the decarburisation of the prior art methods.
As an advantage, the metal also comprises, expressed in weight, a maximum of 0.4% Mn, between 0.1 and a maximum of 2.0% Si, a maximum of 2.0% Al, a maximum of 0.04% P, a maximum of 0.04% S and a maximum of 1% of other alloy elements, the balance being iron and common impurities. Silicon or aluminium in suitable quantities confers an additional inoculating effect to the extent that they both increase the precipitation strength of graphite, thereby reducing the graphitisation time without the presence of residual cementite.
According to a preferred embodiment of the invention, the water or steam granulation step is carried out in the presence of a surfactant.
As an advantage, the water or steam granulation step is preceded by the step of dividing the flow of the molten metal.
The step of dividing the flow of molten metal is preferably carried out by means of a rotating plate.
According to a first embodiment, the rotating plate is a plate made of a material that is not wetted by the molten steel such as zirconia, alumina, boron nitride or syalon.
According to a second embodiment, the rotating plate is a perforated plate.
According to another preferred embodiment, the water or steam granulation step is replaced by a sintering step using iron powder.
According to a third preferred embodiment, the water or steam granulation step is replaced by a step of mechanical manufacturing using wire or sheet.
A second aim of the present invention relates to a steel spheroid projectile or ball with a carbon content between 0.8 and 2.0% C by weight and with a core hardness lower than 100 HV 10, preferably lower than 85 HV 10, implemented by means of the above-mentioned method.
As an advantage, said projectile has a carbon content between 1.2 and 1.8% C by weight.
The projectile is preferably provided with a surface coating to prevent aggregation of the charging.
A third aim of the present invention relates to a cartridge for hunting or target shooting comprising a charging of steel balls such as those described above.